Trans-1,4-polybutadiene is a thermoplastic resin by virtue of its high level of crystallinity. Because trans-1,4-polybutadiene (TPBD) contains many double bonds in its backbone, it can be blended and cocured with rubbers. Even though TPBD is a thermoplastic resin, it becomes elastomeric when cured alone or when cocured with one or more rubbers.
TPBD is normally prepared utilizing transition metal catalysts or rare earth catalysts. The synthesis of TPBD with transition metal catalysts is described by J. Boor Jr., "Ziegler-Natta Catalysts and Polymerizations", Academic Press, New York, 1979, Chapters 5-6. The synthesis of TPBD with rare earth catalysts is described by D. K. Jenkins, Polymer, 26, 147 (1985). However, molecular weight control is difficult to achieve with such transition metal or rare earth catalysts and monomer conversions are often very modest.
Better molecular weight control could be achieved by utilizing an anionic polymerization system to produce the TPBD. There is typically an inverse relationship between the catalyst level utilized and the molecular weight attained when anionic polymerization systems are used. Such an anionic polymerization system is disclosed in U.S. Pat. No. 4,225,690. The catalyst system disclosed therein is based on a dialkylmagnesium compound which is activated with a potassium alkoxide. However, only a minor amount of the polymer produced with such dialkyl magnesium based catalyst systems is TPBD. In other words, the small amount of TPBD produced utilizing such catalyst systems is always accompanied by major amounts of hexane-soluble polybutadiene of mixed microstructure.
Japanese Patent Application No. 67187-1967 discloses a catalyst system and technique for synthesizing TPBD consisting of 75 to 80% trans-1,4-structure and 20 to 25% 1,2-structure. The catalyst system described by this reference consists of a cobalt compound having a cobalt organic acid salt or organic ligand, an organoaluminum compound and phenol or naphthol. Gel formation is a serious problem which is frequently encountered when this three component catalyst system is utilized in the synthesis of TPBD. Gelation is a particularly serious problem in continuous polymerizations.
U.S. Pat. application Ser. No. 07/636,861, filed on Jan. 2, 1991, discloses a catalyst system which can be utilized in the polymerization of 1,3-butadiene monomer into trans-1,4-polybutadiene, said catalyst system being comprised of an organocobalt compound, an organoaluminum compound, a para-alkyl substituted phenol which contains from about 12 to about 26 carbon atoms, and carbon disulfide. By utilizing such a catalyst system TPBD can be synthesized in a continuous process with only minimal amounts of gel formation.
It has been discovered that high trans polymers can be utilized to improve the characteristics of tire tread rubber compounds. By utilizing high trans polymers in tire tread rubber compounds, tires having improved wear characteristics, tear resistance, and low temperature performance can be made. Such high trans polymers include, trans-1,4-polybutadiene, trans styrene-isoprene-butadiene terpolymers, isoprene-butadiene copolymers, and trans-styrene-butadiene copolymers.